Log Scaling - The Ontario Log Rule
S&W Report / Winter / Spring 2000, Vol 18
The Ontario Rule is just one method used in the forest industry to measure (scale) the volume of a sawlog in FBM prior to processing. The unit of measurement is expressed in board feet (FBM). One board foot measures 1 inch by 12 inches by 12 inches. The forest industry uses a thousand board feet (MFBM) as a convenient unit of measure.

Other scaling methods used in the industry include the Doyle Rule, Schibner Rule, International Rule, and the Cube-Grade method used by the Ontario Ministry of Natural Resources. With the exception of the Cube-Grade method, all these scaling methods are measured in imperial units (FBM).

Volume Calculation

The formula used in the Ontario Rule to determine the volume of a log takes into consideration the allowances for kerf and slab lost during the sawing process. The formula to calculate the net volume is as follows:

FBM = (0.55D2 - 1.2D) x L/12

Where D = the top diameter of the log measured in inches and L = the length of log measured in feet (all calculations are be completed to three decimal places). Table #1 shows the volumes in fbm for a variety of diameters and log lengths.
Table 1: Volume Table (Ontario Rule)
Log Length in Feet
8 10 12 14 16
Diameter in Inches Inside Bark at Small end of Log
Volume in Board Feet
4 3 3 4 5 5
5 5 6 8 9 10
6 8 10 13 15 17
7 12 15 19 22 25
8 17 21 26 30 34
9 22 28 34 39 45
10 29 36 43 50 57
11 36 44 53 61 71
12 43 54 65 76 86
13 52 64 77 91 103
14 61 76 91 106 121
15 70 88 106 123 141
16 81 101 122 142 162
17 92 115 139 162 185
18 104 130 157 183 209
19 117 146 176 205 234
20 131 163 196 229 261
21 145 181 217 254 290
22 160 200 240 280 320
23 176 219 263 307 351
24 192 240 288 336 384
25 209 261 314 366 418
26 227 284 341 397 454
27 246 307 369 430 491
28 265 331 398 464 530
29 285 356 429 499 570
30 306 382 459 536 612
31 328 409 491 573 655
32 350 437 525 612 700
33 373 466 559 653 746
34 397 496 595 694 793
35 421 526 632 737 842
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Measuring Procedures

To determine the volume of your log pile, each log must be measured in the following fashion:

Log lengths are measured in full feet. There is an allowance for an extra 6 inches for trimming (known as broomage) at the sawmill for logs measuring up to 15 feet in length. For logs measuring from 16 to 18 feet, the broomage allowance should not exceed 8 inches. Logs are usually processed in 8, 10, 12, 14 or 16-foot lengths, however, logs destined to be sent to a veneer mill will be cut into odd lengths.

Small end diameters of the log are measured inside the bark and recorded in one-inch diameter classes and rounded off to the nearest inch.

Defects

Interior defects are ones that show on the end of the surface of the log. The defect is contained in the heart of the log in a manner that will allow for the sawing of a normal slab and at least one sound board from all sides of the log. Examples of interior defects include heart rot, shake, punk and buttermilk rot. Deductions for interior defects are calculated on the board foot volume basis by applying the formula:

FBM = (diameter of defect in inches)2 x (length of defect / 12) x ¾

Interior defects shown on both ends of the log are referred to as being a continuous defect. The diameter for continuous defects is obtained by averaging the diameter of the defect at both ends of the log. Defects only shown at one end of the log are called partial, and are considered to extend halfway through the length of the log.

Example #1 - a 20-inch log 16 feet long has a 10-inch continuous heart defect. The gross volume equals 261 fbm. A 10-inch interior defect reduces the volume by 100 fbm. The net volume of the log is 161 fbm.

Example #2 - a 23-inch log 10 feet long has a 6-inch partial heart defect. The gross volume for a 10-foot log is 219 fbm. The volume of the defect is 12 fbm. The net volume of the log is 207 fbm.
Table #2 - Deductions for Exterior Defects (Ontario Rule)
Top Diameter
(Inches)
 Percentage Defect
10% 20% 25% 33 1/3% 50% 66 2/3%
4





5


1 1 1
6

1 1 1 1
7
1 1 1 2 2
8
1 1 1 2 2
9
1 1 1 2 3
10
1 1 2 3 3
11 1 1 1 2 3 4
12 1 1 1 2 3 4
13 1 1 2 2 3 4
14 1 1 2 2 4 5
15 1 1 2 3 4 5
16 1 2 2 3 4 6
17 1 2 2 3 5 6
18 1 2 2 3 5 7
19 1 2 2 3 5 7
20 1 2 3 3 6 7
21 1 2 3 4 6 8
22 1 2 3 4 6 8
23 1 2 3 4 7 9
24 1 2 3 4 7 9
25 1 3 3 4 7 10
26 1 3 3 5 8 10
27 1 3 3 5 8 10
28 1 3 4 5 8 11
29 1 3 4 5 8 11
30 1 3 4 5 8 12
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Exterior Defects appear on the circumference of a log and can be found under a number of different conditions. These include a defect on the surface of the log that extends towards the circumference enough to prevent the sawing of a slab and sound board from all sides; lightning scars and seams extending along the log; and a crook or sweep in a log.

Deductions for exterior defects are made on a percentage basis. The percentages are categorized as 10%, 20%, 25%, 33 1/3%, 50% and 66 2/3%. Table #2 shows the appropriate diameter deduction based on these percentage groupings that apply to all log lengths.

Example #1 - a 16-inch log containing a 25% crook would have a reduction in diameter of 2 inches. The net diameter of the log would be 14 inches.

Example #2 - a 22-inch log has a fire scar at the butt end which requires an estimated 10% reduction. The gross diameter will be reduced by 1 inch for the fire scar to a net diameter of 21 inches.

Culls are logs in which over 2/3 of the board foot volume is defective.

Log Grading Specifications

The Ontario Rule is used to estimate the net volume of a log, however, the quality of the log must also be determined. Both the size (volume) and quality (grade) of your logs will dictate their real financial value. A number of factors must be considered in determining the grade of a log. Some of the factors used in determining the grade of a log include:
-
 The position of the log in the tree (i.e. butt or upper);
-
 The minimum top diameter of the log (grade 1 log - butt 12'+ or upper 14'+);
-
 Exterior defects (crook, sweep, seams or heavy branching); and
-
 Interior defects (heart rot, shake or heart check).
There is no industry standard regarding the method to be used in determining the volume or grade for sawlogs. The owner-operator of each mill can use the method of their choice. The woodlot owner should request the mill's log-making specifications prior to commencing cutting activities in their woodlot.

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